The add-on allows you to create reminder on any JIRA issue screen. For example when resolving an issue you can create a reminder for QA team to verify resolution. In order to create a reminder during an issue transition you first need to add "Create Reminder" custom field to workflow transition screens.
You can find more details on how to add a custom field in JIRA's own documentation. On custom field dialog select "Create Reminder" custom field.
Associate field with workflow transition dialog you want and you are done. Since this is also a standard JIRA operation you can find details of associating a custom field with a screen in standard JIRA documentation.
When you perform a workflow transition, for example making an issue "Done", you will now have an option to create a reminder if you check "Adding/Editing Reminders" option. Please refer to "Creating Reminder" section for details of field.